看來又是 GTK+ 的 bug / feature, 換一層顯示大概就有透明了。
星期四 十一月 06, 2008
From gnome-2.24, iiim-panel shows a gray border in gnome-panel, when
gnome-panel is set with "Solid color" or "Background image". It turns
out that the 2nd nested window (GtkDrawArea, or GtkEventBox) does not
inherit the style from its ancestor containers. I tried to use gdk_window_set_back_pixmap to make it transparent, but failed.
egg_tray_icon \_ event_box \_ hbox \_ drawing_area